The present invention relates generally to an apparatus for transmitting an S-video signal from a video source over a transmission line having a length typically detrimental to the S-video signal and the processing of the S-video signal for enhancing the signal within selected frequency bands for overcoming transmission line loss. More particularly, the invention relates to the processing of chrominance and luminance signal components for enhancing brightness and resolution typically reduced through the transmission line loss.
As is well known in the television and video industry, a composite video signal includes luminance and chrominance components that are combined or encoded, such as in an American standard signal established by the National Television System Committee (NTSC) of the Federal Communications Committee (FCC). By combining these signals, the quality of the final signal available is reduced because precise decoding for viewing a final image has yet to be achieved. The original frequency response parameters for the video signal specified in the NTSC standard required a total frequency response of 4.5 MHZ as illustrated by way of example with reference to FIG. 1. This includes all picture information for horizontal and vertical scanning, flyback for retrace, and sound. This does not take into account the 1.5 MHZ separation distance required between TV channels. The total system requires 6 MHZ. Further, the frequency response of a system is directly proportional to the amount of resolution available on an associated monitor screen. The higher the frequency, the smaller will be each picture element, and the better the resolution.
With the introduction of color, an additional bandwidth of approximately 1 MHZ was required. The standard required this color or chrominance information to have a frequency range between approximately 3 MHZ and 4.5 MHZ. This provided the color bandwidth needed but at the same time limited the upper end frequency response for luminance. Such requirements limit picture information changes in a xe2x80x9csuper detailxe2x80x9d area. By way of further example, a frequency response as in FIG. 1 extending to approximately 15 MHZ is applicable for HDTV. More and more, video equipment manufactures are developing new ways of using this standard information within video systems with a goal of achieving improved picture quality. The present invention, as will be herein described, addresses improving the picture information, especially in this super detail area, when transmitted over a long transmission line.
By way of example, instead of using the composite signal to provide picture information directly, the composite signal is processed for providing luminance and chrominance signals on separate signal channels. By separating the luminance and chrominance signals, interference and difficulty in controlling the picture detail is dramatically reduced.
As is well known in the art, chrominance (chroma) defines the color information in a composite video signal and describes the hue and saturation of a picture, but not the brightness. The brightness and contrast are described by the luminance component of the signal. The luminance is a monochrome component of a color video signal. Compatible color systems present luminance values of an image in a signal which is essentially that of an equivalent monochrome transmission. The hue and saturation values of the image are transmitted on a color sub-carrier wave located within a frequency band of the luminance signal, and is recoverable from it. By arranging scanning frequencies to be rigidly tied to the color sub-carrier frequency, the spectrum components of the chrominance signal (hue and saturation) are interleaved in frequency with those of the luminance signal. Therefore, since the chrominance signal contains essentially no luminance information, it has no noticeable effect on monochrome reception. The chrominance signals are recovered in color receivers and are combined therein with the luminance signal to recreate the primary color video signals such as in the FCC/NTSC composite color signal.
Separating the luminance and chrominance signals improves picture quality but it is well known that such luminance and chrominance information is limited to transmission over cable lengths within a few meters before significant degradation of the signal information makes the picture unacceptable to the user or viewer. There is a need in the industry to provide for the transmission of such luminance and chrominance signal information over varying lengths of transmission cable or lines with the further feature of permitting the user to adjust the picture quality, determined by luminance and chrominance, to the taste of the user. There is a further need to be able to adjust for varying lengths of cable or transmission lines selected for the convenient of the user.
The frequency response for the video signal after being transmitted over significant transmission line lengths plays an important role. By way of example, consider the sensitivity of the video signal for systems using a frequency band width of 4.5 MHZ with different carrier video components needed to make up a complete video signal used by television. Picture, synchronization, sound, and color makes up this composite format. The scan rate frequency used is 15,750 Hz. During operation, this scanning frequency sweeps a dot along the face of a monitor picture tube varying the intensity of the dot proportional to the amplitude of the signal. The resolution defined by the quantity of dots that are displayed is directly proportional to the frequency response of this composite video envelope. In other words, a frequency response having higher frequencies will result in a sharper video image on the monitor.
By way of example, should the band width of the composite video signal be 3 MHZ, the actual resolution would be determined as follows: First, determine the time it takes for one line of picture information to travel from the left side of a monitor screen to the right side before retrace occurs. Since the scan frequency is 15,750 Hz, the time to complete one scan can be calculated by finding the reciprocal of the frequency, which is 63.3 micro seconds. Next, determine the time it takes for 3 MHZ to produce one picture element or 2 dots. One element is made up of movement through 2 dots. The same rule follows, take the reciprocal of 3 MHZ, which is 0.333 micro seconds. That is the time it takes to produce one element or 2 dots. This means that approximately 6 dots appear in one microsecond. Finally, multiply the 6 dots times the scan rate of 53.5 micro seconds (63.5 less 10 used for retrace) which provides the number of dots to go across the whole screen. The result is about 314 dots that can be scanned with one sweep trace at 3 MHZ.
Consider a video signal having a response with a frequency band width of 4 MHZ and redetermine the resolution. The reciprocal is 0.25 micro seconds. Again, it takes 2 dots to make up one element. That is 8 dots times 53.5 micro seconds and you get 426 dots of resolution. It is thus quite obvious how much frequency response can change the picture detail. More picture elements provide better the picture detail, but more picture elements demand more frequency bandwidth.
When color television was introduced in the 1950""s, a certain amount of the video bandwidth was sacrificed in order to accommodate the need for color information. By colorplexing (colorcoding) or providing a matrix during the transmit procedure of video, the colors retrieved from the three color camera, (red, green, blue) are encoded to produce 2 sets of color sidebands, one labeled I and the other Q. These 2 color mixtures are transmitted together, one being 90 degrees out of phase with the other. Since the human eye sees fine picture elements in black and white, color need only satisfy viewing the large objects. The amount of bandwidth needed for I and Q was then limited to only 1.5 MHZ, as illustrated by the frequency range for the chrominance signal portion of the video signal of FIG. 1. This frequency band or range of frequencies is nowhere near the range needed for black and white information.
An interesting phenomenon occurs within the video envelope. Sidebands were created by the black and white information also. These sidebands produced empty clusters around the harmonics of the scan rate 15,750 Hz. As a result, voids are produced between these side band clusters that are not used. The decision was made to use one of these empty areas with a high enough harmonic frequency to keep most of the chrominance information away from the luminance signal. The frequency picked was 3,579,545 Hz (3.5 MHZ as illustrated with reference to FIG. 1). The high frequency response of the video luminance signal is therefore limited to approximately 3 MHZ. For television in the 50""s, this was adequate. Keep in mind that when these decisions were being made, commercial television was primarily transmitted in black and white. Typical television sets had thirteen inch diagonal picture tubes or viewing screens. The need for xe2x80x9csuper resolutionxe2x80x9d did not have the importance that color transmission has as of this writing. As television became more and more improved, improved bandwidth was desired. One way this could be accomplished was by separating the Chrominance and the Luminance signals and transmitting them separately. When these signals arrive at a receiving end, they can be routed without disturbing the original video envelope bandwidth. This procedure re-established the original video frequency response of 4.2 MHZ available from the NTSC system. This format was called Y/C or S. It is well known in the industry that high quality television products such as video monitors, laser disk players and DSS devices offer S video in a standard package.
However, the cable or transmission line used has a direct relationship to the signal produced and ultimate picture quality as viewed by the user. The S type video transmission cable that became a standard was small and produced an inadequate impedance match, high capacitance, and a high resistance which caused the video output, frequency response, and chrominance to be limited. The industry immediately discounted the idea of using the S-video signal because of the poor reliability and significant signal line loss. As laser discs, DVD, and DSS became more available, the need for improved resolution grew stronger. The S-video signal carried the high frequency component information for providing a picture pleasing to the viewer but it is this high frequency information that is quickly lost or degraded within relatively short cable transmission runs. There remains a need for providing S-video signal transmission over practical cable lengths. Further, by providing such a transmission capability, one skilled in the art will be better prepared to deal with HDTV and the improvements provided by digital television.
The present invention, an S-video signal processing apparatus satisfies the ever increasing need for a quality color signal. By amplifying and shaping the frequency response rather than simply amplifying it as has been seen in the industry, an S-video signal can be transmitted over generally great lengths of transmission cable and provide a quality picture pleasing to the viewer. Phase problems associated with typical feedback signal processing is corrected.
It is an object of the present invention to provide enhancement of an S-video signal, containing luminance and chrominance (chroma) information, for delivering an enhanced S-video signal over a transmission line selected by the viewer for viewing on a monitor displaying a desirable image. It is further an object of the invention to provide a manually adjustable enhancement to the S-video signal for delivery of the S-video signal over lengthy transmission cables without signal degradation. It is further an object of the invention to provide transmission of a source video signal to a plurality of monitors while permitting adjustment of each signal for providing a pleasing picture on each monitor regardless of differing transmission line lengths between monitors and the video source.
The present invention is directed to S-video signal compensation for providing adjustable electronic compensation to maintain picture quality of an S-video signal transmitted over lengths of S-video cable, and provides embodiments, by way of example, with NTSC and HDTV video signal bandwidths. One preferred embodiment of the present invention provides a single S-video input and single S-video output with a dedicated set of controls including a chroma gain, luminance, gain, and cable compensation control, thus allowing for a precise and customized calibration of the S-video signal. An alternate embodiment provides for multiple inputs and outputs. S-video signal compensation provided is useful for broad bandwidths needed in a typical video signal envelope. With a frequency response from DC to 20 MHZ, the S-video compensators of the present invention are capable of increasing gain with little or no distortion or noise either in luminance or chroma signal information. The embodiments of the present invention, herein described by way of example, provide a full complimentary output stage eliminating the need for coupling capacitors that could cause phase and frequency errors. Plus and minus power supplies feed highly accurate DC regulators, providing perfect operation even with AC voltages as low as 80 volts. Large filter capacitors eliminate the 120 hertz hum before regulations starts. Channel separation is achieved and is more critical than for audio signals because of the high probability of beating interference and oscillation. A unique slope amplifier is incorporated within the luminance stage to correct for any frequency degradation caused by connecting cables.
These and other objects, features, and advantages of the invention are provided by a video signal loss compensation apparatus useful in transmitting an S-video signal through a transmission line from a video source to a video monitor comprises luminance signal amplifying means operable with an S-video signal input means for amplifying a luminance signal within a luminance signal channel, parallel signal processing means operable with the luminance signal amplifying means for modifying the luminance signal delivered through the luminance signal channel, wherein the parallel signal processing means is operable within a frequency range lying within the video signal bandwidth including upper video bandwidth frequencies, and luminance signal adjusting means operable with the amplifying means for modifying a preselected band of frequencies within the video signal bandwidth for transmission of the luminance signal, wherein the preselected band of frequencies includes frequencies within a lower video band width frequency range.
In one preferred embodiment, the parallel signal processing means comprise luminance signal feedback means providing a positive feedback signal within the luminance signal channel for adjustment of the amplified luminance signal, the luminance signal feedback means providing a positive feedback signal having a frequency response within the video signal bandwidth. In another preferred embodiment, the parallel signal processing means comprise a compensation amplifier for altering the frequency response of the luminance signal, and wherein the luminance signal adjusting means is operable in a series connection therewith.
Embodiments further include S-video signal input means for receiving a luminance signal and a chrominance signal, each having a frequency response defined within a video signal bandwidth, the luminance and chrominance signals providing an S-video signal representative of a video source picture signal. Further, chrominance (chroma) signal amplifying means is operable with the S-video signal input means for amplifying the chrominance signal within a chrominance signal channel, chrominance signal amplifying means for amplifying the chrominance signal within a chrominance signal channel electrically separated from the luminance signal channel, and S-video signal output means operable with the luminance and chrominance signal channels for transmitting the amplified luminance and chrominance signals over a transmission line to a monitor, wherein the transmission line provides separate luminance and chrominance signal conductors.
By way of example for one embodiment, with a positive feedback signal across the amplifier and a high frequency band pass filters, both adjustable, a user can adjust the video receiver to the same quality one could achieve by using a one meter length of the highest quality cable. Further, basic off-the-shelf cable such as standard shielded telephone cable is successfully used to transmit the video signal. A positive feedback resistor (potentiometer) and capacitor network provides an increased signal amplitude to achieve a desirable high frequency response while a second resistor (potentiometer) and capacitor network produce an upper to mid range slope or adjustment in frequency response to accommodate white balance and detail for a strong mid to upper frequency response driver offering signal nourishment needed to produce a preferred S-video signal for a vivid picture after experiencing transmission line loss.
By cloning a preferred embodiment, as is described in the detailed description section of this specification, an S-video distribution system is provided. The input of the distribution system is coupled directly through input gain potentiometers for both chrominance and luminance signals. The constant value of the potentiometers used offer a fixed input impedance satisfying the incoming source. Since both luminance and chrominance are now adjustable, each monitor can be adjusted for luminance, chrominance, and resolution, keeping each monitor in check for quality. An additional summing amplifier is provided to reproduce a composite output to meet a need for this format in anticipated field use. Two amplifiers having unity gain buffer the incoming S signal for providing a slave output to run additional distribution systems. This gives the user multiple channels, slave channels, and composite channels. By using a one ampere power supply, the need for extra supplies for each system is eliminated.
One embodiment of the present invention comprises a luminance signal amplifier for amplifying an input luminance signal provided by a video source, the luminance signal amplifier providing an amplified luminance signal, a bandpass filter operatively connected to the amplifier for receiving the amplified luminance signal, the filter coupled with the amplifier for providing an output luminance signal having an enhanced signal frequency range lying within the video signal bandwidth, and a chrominance signal amplifier for amplifying an input chrominance signal provided by the video source, the chrominance signal amplifier providing an amplified output chrominance signal, wherein the output luminance signal and output chrominance signal in combination provide an S-video signal having electrically isolated luminance and chrominance signal portions. In one embodiment of the present invention, a luminance feedback network is operatively connected across the luminance signal amplifier for providing a feedback signal for amplifying by the luminance amplifier and combining with the amplified luminance signal for enhancing the luminance output signal frequency response. The feedback network affects the luminance output signal within a frequency range including upper video band width frequencies. The filter affects the luminance output signal within a lower video band width frequency range. The luminance output signal thus has a frequency response including the frequency signal bandwidth.
A method aspect of the invention includes transmitting an S-video signal provided by a video source to a monitor and comprises the steps of receiving a luminance signal within one signal channel, wherein the luminance signal includes a frequency response defined within a video signal bandwidth. The luminance and chrominance signals provide the S-video signal representative of a video source picture signal. The luminance signal is amplified within the luminance signal channel, wherein a broad band of frequencies is selected within the video signal bandwidth for transmission within the luminance signal channel. The luminance signal channel is electrically isolated from a chrominance signal channel for transmission over a transmission line having separate luminance and chrominance signal conductors. The luminance signal is amplified within a luminance signal channel and modified with a parallel processing signal operable with the luminance signal for modifying the luminance signal delivered through the luminance signal channel, wherein the parallel signal is operable within a frequency range lying within the video signal bandwidth including upper video bandwidth frequencies. The luminance signal is adjusted within a preselected band of frequencies within the video signal bandwidth for transmission of the luminance signal over a transmission line. The preselected band of frequencies includes frequencies within a lower video band width frequency range.
In one method of the present invention, a feedback signal is provided within the luminance signal channel for adjustment of the amplified luminance signal, the feedback signal having a frequency response within a frequency range lying within the video signal bandwidth, the feedback signal frequency response covering a frequency range including upper video band width frequencies, and wherein the broad band selecting step includes selecting frequencies within a lower video band width frequency range.
The present invention includes a correction system for handling the tough demands for quality video signal transmission associated with S-video applications. With a video amplifier having a gain of 6 dB and varying luminance signal frequency response over a particular frequency range using a bandpass filter, an output luminance signal is provided that has gain and adjustment sufficient for delivering a quality signal over a transmission line to a monitor. With a preselected gain and preselected frequency response, it is now possible to transfer a luminance signal through transmission cable having lengths ranging upwards to 150 and 300 meters while being able to maintain excellent picture quality.